GLX DRI3 GPU Offloading Lands In Mesa

The GLX DRI3 GPU offloading support has landed in Mesa with a solution that's superior to the DRI2 GPU offloading model.

Axel Davy's work for DRI3 offloading in Mesa hit Git last night and he explains with the commit for this feature to use secondary GPUs for rendering:

The differences with DRI2 GPU offloading are:

a) There's no logic for GPU offloading needed in the Xserver

b) for DRI2, the card would render to a back buffer, and the content would be copied to the front buffer (the same buffers everytime). Here we can potentially use several back buffers and copy to buffers with no tiling to share with X. We send them with the Present extension.

That means than the DRI2 solution is forced to have tearings with GPU offloading. In the ideal scenario, this DRI3 solution doesn't have this problem.

However without dma-buf fences, a race can appear (if the card is slow and the rendering hasn't finished before the server card reads the buffer), and then old content is displayed. If a user hits this, he should probably revert to the DRI2 solution (LIBGL_DRI3_DISABLE). Users with cards fast enough seem to not hit this in practice (I have an Amd hd 7730m, and I don't hit this, except if I force a low dpm mode)

c) for non-fullscreen apps, the DRI2 GPU offloading solution requires compositing. This DRI3 solution doesn't have this requirement. Rendering to a pixmap also works.

Michael Larabel is the principal author of Phoronix.com and founded the web-site in 2004 with a focus on enriching the Linux hardware experience and being the largest web-site devoted to Linux hardware reviews, particularly for products relevant to Linux gamers and enthusiasts but also commonly reviewing servers/workstations and embedded Linux devices. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics hardware drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated testing software. He can be followed via Twitter and Google+ or contacted via MichaelLarabel.com.